The present invention relates to robots and more particularly to position and velocity feedback systems used in the position and velocity loops in robot controls.
In the operation of robot control systems, individual axis position and velocity parameters are placed under closed loop control. Microprocessors may be employed to make control loop calculations and perform other functions for the robot control. In any case, motor velocity and axis (motor) position signals are required for control loop operation.
Position feedback may be absolute or it may be incremental, and it is the latter type to which the present invention relates. Velocity feedback may be generated by tachometers and/or it may be derived from position feedback as it is in the present case.
The most common type of incremental position feedback encoder is based on optical technology. The optical encoder typically employs optical transmitters and receivers with a rotating disc having transparent slits. Two phase signals and an index signal are generated by the encoder.
The phase signals are offset from each other by 90 degrees in a quadrature encoding scheme such that they can exist in four different states. State transitions enable incremental position change and direction of position change to be detected.
The index signal may occur once every optical disk revolution thereby providing a mechanism for verifying accumulated incremental position change from the two phase signals.
In implementing digital robot control, the amount of digital circuitry required with conventional prior art approaches for conversion of the encoder phase signals to incremental counts for access by a microcomputer is extensive. Moreover, although derivation of velocity from position change is typically acceptable at high velocities, inadequate velocity resolution is achieved at low velocities with conventional conversion circuitry. Better low velocity resolution can be achieved with increased resolution of the encoder, but the extent of improvement through this alternative is limited and in any case results in substantially increased product cost.